New Discoveries Provide Insight into Protective Immune Responses to HIV
Report from the 2009 Keystone Symposia on Prevention of HIV/AIDS and HIV Immunobiology
The 2009 Keystone joint symposia on "Prevention of HIV/AIDS" and "HIV Immunobiology: From Infection to Immune Control" held in the rarified atmosphere of Keystone Resort in Colorado from March 22-27 marks the 25th anniversary of its first HIV/AIDS meeting.
The keynote address by Nobel laureate, Dr. Francoise Barre-Sinousi emphasized the need for rethinking future vaccine strategies by using an integrated approach that combines knowledge of innate and adaptive immunity as well as the early pathogenic events that occur during HIV infection. She reminded us that the contribution of T cell activation in infection should not to be dismissed when considering prevention and therapeutic strategies.
Genetic Makeup Influences HIV response
In a scan for genetic factors that allow some people to naturally control their viral load for long periods of time, a group known as elite controllers, the human leukocyte antigen (HLA) class I genes, HLA-B*57 and -B*27 in particular, have the strongest association with control of viral replication (J. Virol. 83: 1845, 2009). In addition, cytosine at -35 promoter region upstream of HLA-Cw gene marker associates with a lower viral load. Mary Carrington of the National Cancer Institute noted that 64.4% of controllers express either HLA-B*57 or -B*27 or -35C region or a combination of KIR3DS1 and HLA-Bw4-80I or KIR3DL1 and HLA-Bw4-80I. All of these genes play a role in antigen presentation and recognition. High levels of HLA-C expression appear to protect against HIV infection. Bruce Walker at Massachusetts General Hospital highlighted that among elite controllers the protective HLA alleles select for mutations in HIV that cripple the virus’ ability to multiply (J.Virol. 83: 3407, 2009).
Cells exhibit fragments of pathogen-derived proteins on their surface through HLA class I molecules. Previously only T cells were thought to recognize these complexes, but it now appears that natural killer cells, a key component of innate immunity, engage these complexes via killer immunoglobulin-like (KIR) receptors on their surface. Work by Galit Alter of Massachusetts General Hospital supports the concept of specific NK-peptide associations. She showed that in HIV-infected subjects KIR2DL2 allele can drive mutations at position 71 or 74 in Vpu protein, which allows the altered Vpu protein to evade recognition by NK cells. Mary Carrington suggested that increased HLA C levels not only may enhance foreign peptide fragment presentation but also HLA-C and KIR2DL2 engagement in the NK cell maturation process may “license” NK cells to recognize HIV-infected cells as foe and kill them.
Quality of Responses Influence Disease Outcome
Upon simian immunodeficiency virus (SIV) infection some monkey species (African green monkeys, AGMs) do not progress to an AIDS-like illness, while others (pig-tailed macaques, PTMs) succumb rapidly. Joseph McCune of the University of California, San Francisco along with Christopher Miller of the University of California, Davis found that infected PTMs quickly lose Th17 cells, a type of CD4 T cells that secrete IL-17, while AGMs maintain these Th17 cells. The loss of Th17 cells creates an imbalance in the ratio of Th17 to T regulatory (Treg) cells leading to a state of heightened immune activation that fuels additional viral replication (PLoS Pathog. 5: e1000295, 2009). Similarly, humans who fail to control their HIV were found to have a mucosal Th17/Treg imbalance. McCune hypothesized that if a treatment or vaccine strategy could prevent this profound early loss of Th17 cells and preserve the Th17/Treg cell ratio, the disease outcome could be significantly improved.
CD8 T cells, also known as cytotoxic T lymphocytes (CTLs) deal a fatal blow to infected cells by releasing perforin and granzymes. Perforin pokes holes in the infected cell’s membrane to allow entry of a serine protease, granzyme B that activates the cell’s built-in suicide program. Using an elegantly designed in vitro assay involving a 6 day stimulation with HIV peptide, Mark Connors of the National Institute of Allergy and Infectious Diseases illustrated how CTLs from elite controllers (ECs) eliminated HIV-infected CD4 T cells by releasing perforin and granzyme B from granules, while CTLs from chronic progressors (CPs) released low amounts of the cytolytic enzymes, thus reducing their power to destroy infected cells (Immunity 29: 1009, 2008). The defect arises due to diminished granzyme B accumulation in the granules. The proliferative and cytotoxic capacity of CTLs from vaccinees who received two doses of Merck’s Ad5 HIV vaccine (HVTN 071) were similar to those observed in CPs potentially explaining, at least partially, the lack of success of this vaccine in a separate trial.
Early Events in Pathogenesis
Knowledge of the early immune responses to natural HIV infection gives insight into the responses that either promote or impair disease resistance. These new insights can be harnessed for designing effective vaccine strategies. Much of our knowledge of the early events that occur upon HIV infection comes from SIV studies in monkeys. Evidence presented by Thomas Hope of Northwestern University suggests that thick mucus secretion in the vaginal cervix considerably reduces the ability of HIV to cross the epithelial barrier by diffusion. Aashley Haase of the University of Minnesota noted that 1-3 days after infecting monkeys by vaginal route, SIV creates small islands of “founder” infected cells. From days 4 to 7 post-infection, plasmacytoid dendritic cells at the site of infection releases cytokines that attract CD4 T cells, thus inadvertently creating a favorable environment for expansion. If the host can utilize mechanisms to prevent both the establishment of the initial cluster and the influx of CD4 T cells, then SIV or HIV may be cleared or controlled.
Lessons for Vaccines
Heterologous prime-boost regime is the vaccine strategy du jour for HIV. But is this strategy effective in providing protection against viral infection? Dan Barouch’s group at Harvard Medical School vaccinated 5 groups of 6 monkeys: Group 1 received DNA prime/Ad5HVR48 boost expressing Gag/Pol/Nef/Env derived from SIVmac251; Group 2 received the same combination as Group 1 supplemented with chemokines, Mip1 and Flt3L; Group 3 got Ad5HVR48 alone encoding the same set of SIV proteins as Group 1 and 2; Group 4 received Ad5HVR48 alone encoding Gag/Pol/Nef but no Env; Group 5 received no vaccine. Ad5HVR48 is a chimeric virus created to dodge pre-existing immunity to Ad5 by swapping the hexon genes of Ad5 for those of Ad48. The DNA/Ad5HVR48 combination regimen elicited a high magnitude of T cell responses, particularly Env-specific CD4 T cell responses, compared to the Ad alone strategy. However, the animals receiving the combination did not lower the setpoint viral load (log 5.51) or improve survival (4/12) after a follow up of 500 days post SIVmac251 challenge. Despite the lower immunogenicity induced, Ad5HVR48 alone not only lowered setpoint viral load (log 4.88) but also improved survival (10/12 animals) post-challenge. As the effects of DNA priming were evident only upon a combined analysis of DNA/Ad groups (Groups 1 and 2) and Ad groups (Groups 3 and 4), Dan Barouch noted that “these data ought to be viewed as hypothesis-generating rather than conclusive evidence.” How well the findings can be generalized remains undetermined.
Great depletion of CD4 T cell population occurs in the gut within 2-4 weeks after HIV infection. To assess vaccine and treatment efficacy intestinal CD4 T cell numbers need to be monitored, but sampling intestinal tissues in human clinical trials entails a difficult and expensive procedure, and infections are rarely detected that early. Molecular markers in blood that can serve as prognostic indicators for intestinal T cell loss or restoration would offer a more practical solution. 47 integrin expression on T cells acts as “molecular zip codes” that dictate homing to the gut. Studies in monkeys from Ronald Veazey’s group at Tulane University demonstrated that β7 levels on CD4 T cells can be used to approximate the proportion of gut-homing 47 CD4 T cells in blood. In addition, loss of blood CD4 T cells expressing 7 directly parallels the loss and restoration of intestinal CD4 T cell population in SIV infection. Thus, detecting 7 expression on circulating SIV-specific CD4 T cells is a potential surrogate marker for monitoring intestinal CD4 T cell loss and restitution in vaccine and treatment trials.
Progress on the Antibody Front
Using an assay designed by Monogram Biosciences, Inc., and with the aid of a technology platform developed by Theraclone Sciences, the Neutralizing Antibody Consortium assembled by the International AIDS Vaccine Initiative (IAVI) isolated two broadly neutralizing antibodies, PG 9 and PG 16, by analyzing sera from 1800 HIV-infected individuals against a panel of 16 viruses from clades A, B, C, D, and CRF01_AE. Antibody genes were rescued from memory B cells and used to construct and express monoclonal antibodies in appropriate vectors. Of the many antibodies recovered, only PG 9 and PG 16 neutralize a broad range of viruses in the panel and at much lower concentrations than the well-known neutralizing antibodies b12, 2G12, 2F5, and 4E10. PG 9 and PG 16 antibodies bind the native HIV envelope trimer much better than the monomeric forms. Given that soluble CD4 inhibits binding of PG 9 and PG 16 to the envelope and that PG 9 and PG 16 do not compete with b12, a neutralizing antibody that binds to CD4 binding site on gp120, this suggests the possibility that PG 9 and PG 16 bind to a CD4 sensitive conformational epitope. With these insights, investigators hope to construct an immunogen that stimulates a neutralizing antibody response.